1. Field of the Invention
The present invention relates to the field of power converters and, more specifically, power converters equipped with a maximum power point tracking control circuit. Such converters are generally applied to the conversion of power provided by an irregular source. In the context of the present invention, “irregular” power source means a power source providing a power likely to undergo abrupt variations, as opposed to power sources providing a stable or slowly-varying power, as is the case for a battery or for the A.C. supply network. Such sources are, for example, photovoltaic panels providing a power varying according to the lighting, wind engines providing a power varying according to the wind speed, elements of tidal power exploitation providing a power varying according to the wave intensity, etc.
The present invention will be described hereafter in relation with photovoltaic element panels. However, the present invention more generally applies to different power sources for which an automated tracking of the maximum power point is needed to optimize the output in case of a power generation.
2. Discussion of the Related Art
A power converter of the type to which the present invention applies is of static converter type, with its component operating in switched mode (on/off). The input and output voltages may indifferently be D.C., A.C. voltages, or others (for example, pulse voltages). The converter can then be a D.C./D.C., D.C./A.C., A.C./D.C. converter, etc. A currently-used control technique for the switching of the converter semiconductor component(s) is the control by pulse width modulation (PWM) at the turning-off and at the turning on of a power transistor. The width of the pulses for controlling the turning-on of the power transistor is regulated according to the load and to the power required by said load. In the applications of the present invention, the pulse width is further regulated according to the power provided by the power source by tracking, for yield reasons, the maximum power point.
FIG. 1 very schematically shows in the form of blocks a conventional example of a power converter of the type to which the present invention applies. In this example, the converter is a voltage step-up D.C./D.C. converter.
Assume a power source 1 formed of photovoltaic elements PV providing a voltage V which is applied across an inductive element L in series with a PWM controlled power switch 2. In the example shown, power switch 2 is formed of a MOS transistor having a gate receiving a signal CTRL formed of a train of pulses of variable width according to the control orders. The junction 3 of inductive element L and switch 2 is connected to the anode of a free wheel diode D having a cathode connected to a first (positive) electrode 4 of a storage capacitor C. Capacitor C provides, between its electrodes 4 and 5, a regulated voltage Vout or current Iout of D.C., A.C. or other type according to the nature of the load connected between electrodes 4 and 5. Electrode 5 of capacitor C corresponds to a reference voltage, for example the ground, for voltage V of panel 1, for power switch 2, and for the output voltage.
When switch 2 is on (for a MOS transistor, this corresponds to an operation in ohmic mode), diode D is reverse biased. Capacitor C supplies the load connected across terminals 4 and 5. Power is accumulated in inductive element L across which is applied voltage V provided by the photovoltaic panel 1. When transistor 2 is off, the power stored in inductance L is transferred to capacitor C by diode D. The operation of a pulse-width modulated power converter is well known and will not be detailed any further. Various types of power switch assemblies are known, according to whether the converter is a step-down, step-up, or step-up/step-down converter.
When the voltage source providing voltage V is irregular, a maximum power point tracking control circuit (MPPT) 10 is generally used. Such a circuit has the function of modifying the width of the pulses for turning on switch 2 according to the variations of the power provided by power source 1. At its input, circuit 10 thus receives a signal (for example, a voltage) proportional to power P provided by source 1. In the example of FIG. 1, power P is obtained by means of a multiplier 7 of a current measurement I in the photovoltaic elements by a measurement of voltage V across panel 1. Circuit 10 provides a two-state signal Q intended to increase, respectively, decrease, the width of the control pulses of switch 2.
Signal CTRL of control of switch 2 is provided by a comparator 11 (COMP) of the converter controlled by circuit 10. This comparator receives, on a first input, a periodic signal provided by a generator 12, for example, a sawtooth of constant high frequency. A second input of comparator 11 receives the output of a ramp generator 13 (RAMP) having its direction inversion (ascending ramp, descending ramp) conditioned by the state of signal Q. The frequency of the sawtooth conditions the frequency, generally constant, of the pulse train of signal CTRL. The instantaneous level provided by generator 13, formed for example of an RC circuit, sets the comparison reference, and thus the pulse duty cycle.
To generate signal Q, circuit 10 comprises two resistive and capacitive circuits 14, 15 (RCF and RCS) forming delay lines of power signal P with different time constants. Circuit 14 is, for example, a high speed circuit as compared to circuit 15, which has a longer time constant. The respective outputs of circuits 14 and 15 are connected to the inputs of a comparator 16 (COMP), the output of which controls a flip-flop 17 (T) providing signal Q. Hereafter, Q will indifferently be used to designate the forward (non inverted) output terminal of flip-flop 17 or the signal present on this terminal. Flip-flop 17 is a flip-flop with no clock signal. It is, for example, a JK-type flip-flop assembled as a so-called T-type flip-flop.
The structure and operation of a circuit such as shown in FIG. 1 is perfectly well known. An example of such a circuit is described in article “Step-Up Maximum Power Point Tracker for Photovoltaic Arrays” by Ziyad Salameh, published in the proceedings of the American Solar Energy Society Conference of Jun. 20 to 24, 1988, pages 409–414. Its operation will briefly be reminded hereafter.
The examination of the slow and fast variations of power P provides an image of the derivative of this power. Due to the time constant difference of RC circuits 14 and 15, the output of comparator 16 oscillates. The frequency and amplitude of these oscillations depend on the time constants of the RC circuits. In fact, comparator 16 indicates, according to its output state (high or low) the sign of the derivative of the power. As long as the output of comparator 16 remains in a same state, the output of flip-flop 17 does not switch state. Assuming a state 1 at the input and at the output of flip-flop 17, the resistive and capacitive circuit of ramp generator 13 builds up power. This increases the corresponding input level of comparator 11 and increases the duty cycle of signal CTRL. Assuming that the load receiving voltage Vout is constant, power P will increase to a maximum, then start decreasing along with the increase in voltage V. When the power starts decreasing, the output of comparator 16 switches, which causes a switching of output signal Q of flip-flop 17. Said signal then switches low, which causes the discharge of the RC circuit of ramp generator 13 and a decrease in the duty cycle. The output voltage then starts increasing again. At constant load, the circuit converges towards a maximum power point and oscillates around this point.
This operation is illustrated in FIG. 2, which shows two examples of the course of power P according to voltage V for two lighting quantities received by panel 1. A first curve 21 illustrates, for example, the case of a maximum lighting. As just described, at constant load, the system will oscillate around maximum power point PMM1.
If the lighting of panel 1 changes (for example, by the coming of shadow), characteristic P=f(V) of panel 1 becomes a curve 22 of lower level. This curve also exhibits a maximum power point PMM2. However, the control system shown in FIG. 1 cannot make out a lighting change from an abrupt variation of the load connected at the converter output or from a mere variation around the maximum power point of curve P=f(V) on which stands its operating point. The control system is then lost and may even find itself in a steady state no longer corresponding to the maximum power point. In fact, the circuit diverges towards a minimum load or maximum load state according to the flip-flop state preceding the change of curve P=f (V). The same problem is posed in the case of an abrupt variation in the supplied load.
A first solution consists of choosing very different time constants of delay elements 14 and 15. However, this adversely affects the output because of the significant generated oscillations.
Another known solution consists of forcing the system to start back from the origin of curves P=f (V). It is then started from a very small duty cycle, which is increased to converge back towards the maximum power point of the current lighting curve. A disadvantage of such a solution is that it considerably slows down the control by the lighting of the photovoltaic panel or by the abrupt variations of any power source connected upstream of the system. Further, the differentiation between a maximum power point change (curve change) and a normal variation also poses a problem in terms of detection duration and reliability.
FIG. 3 illustrates an example of characteristic of current I provided by the photovoltaic panel along time at a power curve change of the panel. It is assumed to initially be (times t0 to t1) on a maximum lighting curve (21, FIG. 2). Current I then slightly oscillates around a value Imax, assuming a constant load. A lighting change at time t1 causes a reference loss for the control system. In the example shown in FIG. 3, it is assumed that the system is then restarted at a time t2 subsequent to time t1 after having discovered the system reference loss. It is then converged until a time t3 towards a new maximum power point corresponding to a current Iomb around which the system then starts slightly oscillating.
The amplitude of the oscillations around values Imax and Iomb of course depends on the time constants of RC circuits 14 and 15. The greater the difference between time constants, the larger the amplitude of the oscillations at the output of comparator 16. The faster it is converged towards the maximum power point (duration between times t2 and t3), the greater the oscillation amplitude. However, the greater the oscillations, the more adversely this affects the system output. A compromise must thus be made between output, speed, and stability.
Problems of system convergence after maximum power point changes are posed especially in case of a variable power source. However, even if the input power source is stable a priori as should be the case, for example, for photovoltaic panels used in space (without clouds), such convergence problems may be encountered. Indeed, space infrastructures having more and more complex geometries, shadow areas due to the very structure of satellites may appear. Further, sensors may be partially damaged by dust impact, which leads to the same result.
Another known solution to overcome the disadvantages linked to abrupt variations of the power source is to use a digital circuit. The different operating points are successively memorized to recognize a drift. A digital system however remains slow to isolate a drift from a normal operating variation. On this regard, the larger the amplitude of the accepted oscillations in steady state, the slower the system will be in recognizing a state change due to a change in the power source. Another disadvantage of a digital circuit is that it is in practice limited to frequencies of pulse trains for controlling switch 2 of some hundred kHz. On this regard, an analog control circuit such as that illustrated in FIG. 1 has the advantage of being able to operate at higher cut-off frequencies (on the order of one MHz). This eases the converter integration.